This invention relates generally to thermostatic electric switches and more particularly to means for providing multiple temperature operation.
Thermostatic switches are used for switching of circuits upon the occurrence of preselected temperature conditions. In some applications it is desirable to effect switching upon the occurrence of more than one temperature. While this can be done by providing separate thermostatic switches for each temperature, assuming that there are no space limitations, this involves added expenses due to duplication of parts.
One approach to this problem is shown in U.S. Pat. No. 3,500,277 wherein a thermostatic switch is provided with two bimetal discs each adapted to snap from one dished configuration to an opposite dished configuration at different temperatures controlling separate circuits. A separate motion transfer pin operatively connects each disc to a respective switch. However there are applications where it is desirable to effect actuation of the same circuit at several temperatures which may vary, for example, based on which of several operation cycles is chosen in an application such as a clothes drier with which the switch is used.
It is known for suppliers of appliances using fixed temperature thermostats to mount wire heating units externally to the thermostats in heat conductive relation thereto and use these heaters to modify the effective operating temperature of the thermostat to obtain one or more operating temperatures matched to different cycles provided for this appliance. However locating the heating unit externally of the thermostat is relatively inefficient and results in a system which is slow in responsiveness.
U.S. Pat. No. 3,579,167 shows a thermostatic switch having an internal heater assembly placed adjacent to the bimetal disc and used to separately control the actuation of the disc. Such switches are used for example, for providing "brown-out" protection for compressor motors. The heater assembly includes a ceramic disc with a resistive coating on the face of the substrate which is connected to exterior terminals. While this switch is effective for applications in which several watts of energy are required it is not feasible for lower wattage applications in which it is desired to modify the operating temperature of a disc only a few degrees since so little surface area is available on the ceramic disc for such printed heaters.
In motor protector devices which employ a thermostatic disc adapted to carry motor current, it is a common practice to place one or more wire heaters closely adjacent the disc. These heaters are typically in the 10-20 watt range. For example, as shown in U.S. Pat. No. 2,768,342 one heater is connected in series with the starting winding and a second heater is connected in series with the running winding and both are in series with the thermostatic disc. Normal motor current passing through the heaters and the disc will not be sufficient to raise the temperature of the disc to the point where it snaps to its opposite dished configuration to open the electrical contacts. However an overload in one of the windings will cause increased current flow and heat output from its respective heater which will raise the temperature of the disc until it snaps to open the electrical contacts.
In applications involving thermostatic switches in which the bimetal disc is typically only approximately 0.75 inch in diameter and in which it is desired to thermally bias the disc only slightly, for example in the order of 10 to 15 degrees Farenheit, the use of such heater wire is not feasible. That is, at typical applied voltages of 115 the heater wire would be excessively fine in order to provide sufficient high resistance and would occupy too much space for the thermostatic switches involved which have housings in the order of less than an inch in diameter.
In copending application Ser. No. 670,182 filed Nov. 13, 1984, assigned to the assignees of the present invention, a thermostatic switch having multiple temperature operation is disclosed and claimed in which a thermal biasing assembly is disposed in a recess formed in the housing in heat conductive relation with the thermostatic snap acting disc also located in the recess. The thermal biasing assembly comprises a pair of cylindrical film type resistors of a selected rating with a first lead of each resistor welded to a first connector and a second lead of each resistor welded to a second elongated connector. An assembly having the proper wattage rating for a given application is chosen and dropped into place in the thermostat in an annular shaped groove adjacent the disc seat by sliding the connectors into a pair of parallel extending bores formed in the housing and the connectors are then attached to suitable terminals mounted on the thermostat housing. The connectors may be formed of low thermal conductivity material to minimize heat loss from the resistors.
While thermostatic switches made in accordance with the referenced application are effective for many applications it is found that for other applications where greater thermal biasing is desired and even though the connectors may be formed of low thermal conductivity material there is still excessive heat loss from the resistors through the leads, connectors and terminals.